Beamforming is a signal processing technique used in antenna arrays for directional signal transmission or reception. Spatial selectivity may be achieved through adaptive or fixed receive/transmit beam patterns. The use of spatial selectivity results in improvements that may be referred to as the receive/transmit gain (or loss).
In some implementations, beamforming takes advantage of interference to change the directionality of the array. In many wireless base stations, a plurality of transmitters are present. When transmitting an outgoing signal, the base station controls the phase and relative amplitude of the signal at each transmitter and creates a pattern of constructive and destructive interference in the transmitting wavefront. When a mobile device receives this outgoing signal, information from different sensors is combined in such a way that the expected pattern of radiation is preferentially observed.
Conventional transmit beamforming in wireless systems can be done with either closed-loop or open-loop manners. Open-loop systems are typically used with TDD (Time Division Duplexing) systems. TDD systems do not require channel information feedback. One disadvantage of open-loop system is that they must constantly conduct phase calibration in order to compensate the phase difference between transmission and reception RF chains among multiple transmit antennas. Another disadvantage is that it requires a constant uplink phase reference such as an uplink pilot and this can lead to an excessive feedback overhead. The process of phase calibration is generally costly, and sensitive to radio channel environment.
Closed-loop systems do not require the phase calibration processes that are required by open-loop systems. The lack of a need for the phase calibration process is due in part to the requirement that closed-loop systems have at least one channel feedback to the transmitter. One problem with the use of the closed-loop system is that the feedback creates a significant amount of overhead for the transmitted. In addition, closed-loop systems are sensitive to the feedback channel error due to feedback delay or fast channel variation. Typically, Frequency Division Duplexing (FDD) systems employs closed-loop transmit beamforming scheme. However, a closed-loop scheme can also be applied to TDD systems.
A practical closed-loop transmit beamforming scheme may be based on a codebook design. The closed-loop codebook-based transmit beamforming has been used for a scenario where a base station form a transmit antenna beam toward a single user at a time and at certain frequency. A codebook is a set of pre-determined antenna beams that are known to mobile stations.